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Intraoperative neurophysiologic monitoring: focus on cervical ...

  1. 1. Intraoperative neurophysiologic monitoring: focus on cervical myelopathy and related issues Vincent J. Devlin, MDa,*, Paul A. Anderson, MDb , Daniel M. Schwartz, PhD, DABNMc , Robin Vaughan, PhD, DABNMd a Geisinger Medical Center, Department of Orthopedic Surgery, M.C. 21-30, 100 North Academy Avenue, Danville, PA 17822, USA b University of Wisconsin Hospitals, 600 Highland Avenue, Suite K4-738, Madison, WI 53792, USA c Surgical Monitoring Associates, 25 Bala Avenue, Suite 105, Bala Cynwyd, PA 19004, USA d Neurophysiology Incorporated, 5395 Ruffin Road, #102, San Diego, CA 92123, USA Abstract BACKGROUND CONTEXT: The use of neurophysiologic monitoring during surgical proce- dures for cervical spondylotic myelopathy (CSM) is controversial. PURPOSE: The aim of this article is to review the literature regarding various monitoring tech- niques as applied to the patient with CSM. STUDY DESIGN/METHODS: A systematic literature review. CONCLUSIONS: Neurophysiologic monitoring is a diagnostic tool for assessment of neurologic function during cervical spine surgery. Recording of somatosensory evoked potentials (SSEPs), transcranial electrical motor evoked potentials (tceMEPs), and electromyograms (EMGs) may be useful as these monitoring modalities provide complementary information. Ó 2006 Elsevier Inc. All rights reserved. Keywords: Neurophysiologic monitoring; Cervical spondylotic myelopathy Introduction Recent advances in the field of spinal monitoring have provided a wide array of techniques to assess the functional integrity of the nervous system. These methods have been used both as an adjunct to clinical evaluation and for neu- rologic surveillance during surgical procedures which place the spinal cord or nerve roots at risk of injury. Use of intra- operative neurophysiologic monitoring (IONM) has be- come commonplace in during spine surgery. The purpose of modern IONM is to provide feedback to surgeons and anesthesiologists regarding changes in neural function be- fore the development of irreversible neural injury, thereby permitting intervention to prevent or minimize postopera- tive neurologic deficit [1]. Historically, the Stagnara wake-up test was the first widely used method for spinal monitoring [2]. Although this test provides assessment of gross integrity of motor function during and at the conclusion of a spine procedure, it cannot be administered in a continuous fashion during surgery and is unable to provide information regarding spi- nal cord sensory tract function or individual nerve root function. Perhaps the greatest shortcoming of the wake-up test is its performance at a single point in time during sur- gery; namely following correction. This presumes, there- fore, that spinal cord injury cannot occur at any other time during the case. This temporal delay between the time of insult to detection by a wake-up test not only prevents identification of the specific surgical maneuver responsible for injury, but also delays timely intervention to prevent or minimize neurologic deficit. In addition, clinical manifesta- tion of spinal cord injury does not always present itself in a time-locked manner. It is entirely possible for paralysis to present long after a wake-up test would have been performed. It is now possible to assess the functional integrity of the dorsal sensory and ventral motor spinal tracts and nerve roots continuously and in essentially real-time, from the on- set of anesthesia induction through emergence using somatosensory evoked potential monitoring (SSEP), trans- cranial electrical motor evoked potential monitoring FDA device/drug status: not applicable. Nothing of value received from a commercial entity related to this manuscript. * Corresponding author. Geisinger Medical Center, Department of Orthopedic Surgery, M.C. 21-30, 100 North Academy Avenue, Danville, PA 17822. Tel.: (570) 271-6541; (570) 271-5872. E-mail address: vjdevlin@geisinger.edu (V.J. Devlin) 1529-9430/06/$ – see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.spinee.2006.04.022 The Spine Journal 6 (2006) 212S–224S
  2. 2. (tceMEPs), and intraoperative electromyography (EMG) [3–5]. Despite the routine use of IONM during instru- mented thoracic and lumbar spine procedures in many cen- ters, its application to cervical spinal surgery has not been nearly as widespread. Potential reasons attributed to this discrepancy include: lack of proven efficacy, cost consider- ations, inadequate understanding regarding contemporary techniques which utilize multiple monitoring modalities combined into an appropriate surgical plan, lack of consen- sus as to the appropriate indications for utilization of IONM, and shortage of highly qualified and experienced personnel to provide monitoring services [6]. This article provides an overview and update regarding IONM with a special focus on issues related to the patient with cervical myelopathy. Overview of spinal monitoring techniques Monitoring of spinal cord function Somatosensory evoked potentials Principles. Somatosensory evoked potentials (SSEPs) are cortical or subcortical responses to repetitive electrical stimulation of a mixed peripheral nerve. Typical stimula- tion sites include the posterior tibial nerve (ankle), the pe- roneal nerve (fibular head), and the ulnar or median nerves (wrist). The ulnar nerve is the preferred stimulation site for upper extremity SSEPs because the lower spinal nerve entry between C7 and T1 permits assessment of the entire cervical neural axis. Electrical stimulation applied to a pe- ripheral nerve creates an afferent volley which enters the spinal cord through dorsal nerve roots at several segmental levels, and may ascend the spinal cord via multiple path- ways. The general consensus is that the dorsal or posterior column spinal pathways are the site of primary mediation for SSEPs [7]. Other pathways such as the dorsal spinocer- ebellar tracts and anterolateral tracts may also contribute to early SSEP responses that are used for monitoring spi- nal cord function. Upon ascending the spinal cord, the neural signal enters the medullary nuclei in the brainstem. Because there are no synapses between the peripheral nerve and the brainstem nuclei, subcortical SSEPs are pre- dominantly a reflection of the integrity of spinal cord white matter. The importance of this fact is that an SSEP recorded up to the level of the lower brainstem, is affected on- ly minimally by general anesthetics. However, subcortical SSEPs can be contaminated easily by myogenic artifact in the unrelaxed patient. After synapsing in the medullary nu- clei, the neural signal crosses the brainstem and ascends in the medial lemniscal pathways. It synapses once again in the thalamic nuclei and then projects up to the sensorimotor cortex where additional synaptic interaction may occur. Be- cause these cortical synapses are the sites of action for inha- lational anesthetic agents, the selection of an anesthetic technique that will optimize cortical SSEP responses is cru- cial, as will be discussed in a later section. Parameters of interest. Data including signal amplitude (power) and latency (velocity) are recorded continuously during surgery and compared with baseline and recently ac- quired data. Of these two parameters, amplitude is most rel- evant. It would be extremely unlikely to sustain a spinal cord injury without amplitude changes. However, changes in latency are quite common and are less significant. SSEP data should be constantly updated to control for anesthetic and metabolic changes. Baselines may be altered during the surgical monitoring to reflect the above changes and used comparatively immediately before surgical changes that might affect the neurologic function of the patient (eg, im- mediately prior to distraction, graft placement). Criteria for surgeon notification vary from center to center but in gen- eral include an intraoperative unilateral or bilateral ampli- tude loss of at least 50–60%. Limitations. SSEPs assess directly spinal cord sensory tracts but provide only indirect information about motor tracts. Damage to the spinal cord motor tracts can occur without a concomitant change in SSEPs. In addition, SSEPs may be poorly defined or unrecordable in patients with se- vere myelopathy, spinal cord tumor, obesity, or peripheral neuropathy either alone or in combination. Anesthesia considerations. SSEP recordings are influenced heavily by inhalational anesthetic agents including nitrous oxide. Subcortical SSEPs are most optimal when the patient is chemically paralyzed as muscle artifact is diminished in the recording thus enhancing the quality of the response. Myogenic interference is much less problematic when recording cortical SSEPs. To address these concerns, many spine centers have switched to a total intravenous anesthetic regimen which will be discussed further in a later section. Predictive value. Despite the high negative predictive value of SSEPs for ruling out motor deficit during surgical correction for scoliosis, SSEPs have been less helpful in monitoring other spinal pathologies. In patients undergoing cervical surgery, May et al. [8] reported that SSEPs were 99% sensitive but only 27% specific in identifying neuro- logic deterioration. In addition there remains a small but definite risk of false-negative findings when monitoring pa- tients with preexisting spinal cord compromise, such as my- elopathy or acute spinal cord injury. In such patients, the vascular supply to both the anterior and lateral aspects of the spinal cord supplied by the anterior spinal artery is vul- nerable to hypotension-induced ischemic injury that may not be detectable with SSEP monitoring at all or during the critical time needed to initiate intervention to prevent or minimize neural injury [9]. Transcranial electric motor-evoked potentials Principles. Motor evoked potentials (MEPs) are neuroel- ectric impulses elicited from descending motor pathways 213SV.J. Devlin et al. / The Spine Journal 6 (2006) 212S–224S
  3. 3. including the corticospinal tract (CST), spinal cord inter- neurons, anterior horn cells, peripheral nerves, and skeletal muscles innervated by excited alpha motor neurons follow- ing the transcranial application of a high-voltage electrical stimulus. A low-output impedance electrical stimulator is used to generate a high-volume, short-duration stimulus or pulse train via a series of electrodes placed over various regions of the scalp to excite a selected area of the motor cortex. This results in stimulation of CST axons which course from the cortex through the internal capsule to the caudal medulla. Here, the fibers cross over in the lower lateral brainstem and descend into the lateral and anterior funiculi of the spinal cord. In contrast to white matter me- diated SSEPs, CST axons that originate in the premotor and motor cortex enter the spinal cord gray matter where they interact with spinal interneurons. The axons go on to synapse with alpha motor neurons which innervate peripheral muscle. Lateral CST fibers that synapse in the cervical segment of the spinal cord are arranged medially followed laterally by fibers that synapse in the thoracic, lumbar, and sacral regions, respectively. MEPs can be recorded either from the spinal cord (I and D waves) or directly from muscle (compound muscle action potential [CMAP]). Although MEPs can also be elicited by transcra- nial magnetic stimulation, the technical challenges for recording these signals in the operating room are too great to warrant use, particularly given the simplicity of electric stimulation [10–12]. Parameters of interest. Transcranial electric motor-evoked potentials (tceMEPs) from the CST can be recorded from the spinal epidural or subdural space via a catheter-type electrode or from peripheral musculature. Responses re- corded from the epidural space consist of what is known as a ‘‘D-wave’’ so-called because it represents direct activa- tion of the CST cells. In awake or lightly anesthetized pa- tients the ‘‘D-wave’’ is followed by a series of ‘‘I-waves’’ generated indirectly by cortical synapses. These descending cortical volleys then summate to excite anterior horn cells and spinal alpha motor neurons thereby inducing a com- pound muscle action potential. D waves are advantageous to record intraoperatively during excision of intramedullary spinal cord tumors. However, the requirement of electrode placement either percutaneously or through a laminotomy precludes routine use in most centers, particularly for mon- itoring most common cervical procedures. In addition, D-waves reflect global CST function which is problematic for monitoring the cervical spinal cord because a selective injury to the cervical cord motor fibers that spares the lower extremity fibers may not be detected. Except in extenuating circumstance, therefore, it is both easier and preferable to record myogenic motor responses (CMAP) from upper (control) and lower extremity peripheral muscle. CMAPs may be recorded either from surface electrodes or subder- mal needle electrodes placed over key peripheral muscles. The warning criterion is typically a 75% or more decrease in CMAP amplitude but must be individualized based on a variety of patient-specific factors. Limitations. Transcranial electric motor evoked potentials are compromised in the presence of neuromuscular relaxa- tion. Muscle relaxants should be avoided during critical parts of the procedure and their use limited to low-risk por- tions of the procedure such as during spinal exposure. Anesthesia considerations. Motor evoked potentials are influenced heavily by inhalational anesthetics and require total intravenous anesthesia for reliable recording. Chemi- cal paralysis will prevent elicitation of MEPs. Predictive value. Transcranial electric motor evoked po- tentials are exquisitely sensitive and specific for the diagno- sis of intraoperative cervical spinal cord injury [13]. Additional techniques for monitoring of spinal cord function in special situations H-reflex and F-response. An intraoperatively recorded H- reflex or F-response can be used to augment transcranial motor evoked potentials for rapid detection of acute spinal shock [14]. Both the H-reflex and F-response aid in the in- traoperative assessment of spinal cord systems responsible for the control of complex motor behavior. As such, they provide a model for understanding the mechanisms of spi- nal cord pathophysiology. Severe acute spinal cord injury leading to spinal shock results in suppression of H-reflexes and F-responses due to hyperpolarization of caudal motor neurons presenting within seconds after spinal cord injury. Despite many advantages over tceMEPs, including requir- ing less anesthetic restrictions, the H-reflex and F-responses tend to be highly variable, and appear to be recordable in only approximately 60–70% of pediatric cases and proba- bly less than 40% of adult cases. Yet, these monosynaptic responses serve as an excellent crosscheck and back-up to tceMEPs, when they can be obtained. Neurogenic spinal evoked potentials (NSEP). Among the most controversial techniques used to monitor spinal cord function is the neurogenic descending evoked potential. This presumed orthodromic response is elicited to transoss- eous (spinous process, lamina) or epidural electro-spinal stimulation, and recorded over lower extremity peripheral nerves (eg, popliteal fossae). Initially, it was thought to be mediated within spinal motor tracts. Because of its tech- nical simplicity both for stimulation and recording, as well as minimal negative effects from most anesthetic agents, in- cluding neuromuscular blockade, the so-called neurogenic ‘‘motor’’ evoked potential became highly popular for more than a decade. Unfortunately, myriad research studies have now clarified that both the neurogenic evoked response and SSEPs are actually mediated through common spinal cord pathways [15–18]. A neurogenic response is not a motor 214S V.J. Devlin et al. / The Spine Journal 6 (2006) 212S–224S
  4. 4. evoked potential, but rather, represents antidromic spinal cord somatosensory activity. This has been confirmed in clinical practice by the finding that NSEPs have been suc- cessfully recorded from paraplegic patients demonstrating that this response does not depend on functionally intact motor tracts. Despite the disappointing evidence that the NSEP is a sensory versus motor evoked potential, its use should not be dismissed entirely. There are very rare occa- sions where it is not possible to record an SSEP, tceMEP, or H-reflex, and yet, epidural stimulation at the rostral thoracic level elicits a descending sensory potential that is record- able over the popliteal fossae. Monitoring of nerve root function Electromyography Principles. SSEPs are neither sensitive nor specific for identification of injury to a specific spinal nerve root owing to their multiple nerve root mediation. Electromyographic (EMG) techniques overcome this limitation and can be classified into two categories based on method of elicita- tion: mechanical and electrical. Mechanically elicited EMG, also called spontaneous EMG (spEMG), may be use- ful during the dynamic phases of surgery (during implant placement, nerve root manipulation). Electrically elicited EMG, also called stimulus-evoked EMG (stEMG) or trig- gered EMG (trEMG) may be useful during static phases of surgery. Together these EMG techniques encourage early detection of excessive nerve-root traction, mechanical in- jury, or cortical breach. The stEMG principle for identify- ing cortical breach resulting from placement of pedicle screws is based on the fact that cortical bone has a high re- sistivity (low conductivity) to electrical current flow whereas soft tissue has a low electrical resistivity [19]. The tip of a monopolar probe is touched to the screw shank or hexagonal port, and the electrical current output is in- creased via an electrical triggering device. If there is a cor- tical perforation, the normally high resistance of the intact bony wall will be reduced and the flow of electrical current from the cathode to the anode will take the path of least re- sistance, namely through the breach to the root. As a result, the nerve root will depolarize at a much lower current (!7.0 mA) compared with an intact pedicle (10–12 mA). Subsequently the root will fire and the peripherally inner- vated muscle will contract, and this will be recorded as a compound muscle action potential (CMAP). This screw stimulation technique has been most widely used in the lumbar region [20,21] and has subsequently been adapted to the thoracic region [22–24] and cervical region [25]. Use of spontaneous EMG (spEMG) techniques to detect in- traoperative cervical nerve root mechanical injury or exces- sive traction has been reported [26]. Parameters of interest. Microtrauma to a spinal nerve root provokes ion depolarization, and the resultant muscle or motor unit potential can be recorded from a muscle innervated by that specific nerve root. Abrupt traction of a spinal nerve root or mechanical contact by a surgical in- strument will elicit intermittent EMG ‘‘burst’’ or sustained ‘‘train’’ activity. Gradual traction may elicit a smaller re- sponse or even no response. Simple ‘‘burst’’ activity reflects mechanical contact with a nerve root and is diagnostically meaningless. Train EMG reflects a state of traction, me- chanical irritation, or thermal change (e.g., secondary to cool irrigation). While the occurrence of long-term unre- solved ‘‘trains’’ suggests that the root is highly irritated, it does not infer that a neurologic deficit will result. Limitations. Chronically compressed motor nerve roots have an elevated threshold [27]. Therefore, these chroni- cally compressed roots will not fire spontaneously or with stEMG techniques resulting in false-positive tests. A quiet spontaneous EMG of a chronically compressed nerve root does not mean the root is not undergoing injury by traction or mechanical contact. Also, thresholds for stimulation of normal nerve roots do not apply to chronically compressed nerve roots. Chronically compressed roots must serve as their own control to establish a safe triggered EMG threshold. Anesthesia considerations. All depolarizing and nondepo- larizing paralytic agent must be avoided as they block the neuromuscular junction and preclude muscle contraction, thereby producing a false-negative stEMG test. Predictive value. The predictive value of intensity of screw stimulation needed to elicit myogenic responses and the risk for neurologic injury is considered to exceed 95% based on support from multiple studies in the lumbar spine. It is recognized that physiologic factors can contrib- ute to false-negative results in the setting of chronically compressed nerve roots and metabolic conditions such as diabetes. Direct stimulation of nerve roots at risk is a method which can overcome this problem when clinically feasible. It is also recognized that while low threshold readings from screw stimulation suggest that the path of least electrical resistance is located near a nerve root, it cannot be distinguished whether the cause is a cracked pedicle, thin wall of osteoporotic bone, or an exposed pedicle screw. Transcranial electric motor-evoked potentials (tceMEPs) for assessment of root function Postoperative C5 nerve root palsy may occur after ante- rior or posterior cervical procedures. The prevalence of C5 nerve root injury is reported as high as 12.9% after lami- nectomy [28] and 14.9% after laminoplasty [29]. In many cases paralysis develops 24 hours or more after surgery. Routine intraoperative neurophysiologic monitoring (IONM) of upper extremity SSEPs, dermatomal evoked po- tentials (DEPs), and tceMEPs recorded from hand muscles generally has failed to detect this serious complication. In 215SV.J. Devlin et al. / The Spine Journal 6 (2006) 212S–224S
  5. 5. an effort to reduce postoperative C5 nerve root injury, intra- operative deltoid and biceps tceMEPs in conjunction with spontaneous electromyography monitoring have been utilized [26]. These neurophysiologic methods may play complementary roles for early detection (spEMG) and functional assessment (tceMEPs) of C5 nerve root injury during posterior cervical decompressive procedures. Dermatomal evoked potentials Dermatomal evoked potentials (DEPs) are a monitoring modality whose use has not withstood the test of time and has been replaced by EMG and MEP techniques for moni- toring nerve roots. DEPs were initially advocated as a mo- dality for assessing adequacy of nerve root decompression. The functional integrity of an individual nerve root was as- sessed by stimulation of a dermatomal field and recording an afferent evoked potential over the scalp similar to that described for mixed nerve SSEPs. DEPs are limited to monitoring of a sensory nerve root and are most useful in nonmyelopathic patients with acute radiculopathy of less than 3–6 months duration. They are not particularly sensi- tive for instantaneous recognition of sharp root injury as oc- curs with placement of bone screws. In addition, DEPs are highly contaminated in the unrelaxed patient. Because MEPs are much more important for guarding the spinal cord and require avoidance of muscle relaxation, and given the highly questionable reliability and validity of DEPs, there is little place for this modality in current practice [30]. Brachial plexus monitoring A tangential benefit of IONM is the ability of SSEPs or tceMEPs to identify impending brachial plexopathy or ul- nar nerve neuropathy secondary to malpositioning. Inter- mittent monitoring of ulnar nerve SSEPs recorded either directly from the brachial plexus (Erb’s point) or cervical spine, coupled with tceMEPs recorded over deltoid, exten- sor carpi radialis, and intrinsic first dorsal interosseous muscles, are highly effective for identifying emerging bra- chial plexopathy or ulnar neuropathy. Numerous reports at- test to the efficacy of IONM for this purpose [31–33]. Pathophysiology of spinal monitoring changes In spinal surgery, neurologic complications usually are secondary to contusion (eg, mechanical trauma), direct dis- tortion of neural elements (such as during deformity reduc- tion), or ischemic insult. By convention, all evoked potentials are evaluated in terms of measured amplitude (voltage), latency (time), and morphology (shape). If injury occurs, from any cause, a cascade of changes involving so- dium, potassium, and calcium channels occurs [34]. This cascade causes blockage of axonal transmission which leads ultimately to an uncoupling of oxidative phosphoryla- tion, thereby precluding adenosine triphosphate production. The net result is loss of cellular function and structural integrity which manifests as a voltage drop in evoked po- tential amplitude, not a prolongation of latency. Other than latency shifts associated with increased concentration of in- halational or intravenous agents, lowering of core body or limb temperature or perhaps hypercarbia, evoked potential latency rarely changes in the absence of an amplitude loss. Therefore, reliance on the 10% latency prolongation rule commonly used to define a significant SSEP change will create an excessive number of false-positive alerts. Spinal cord contusion typically results in a transient spi- nal cord conduction block resulting in marked amplitude suppression (50–75%) of SSEPs or tceMEPs which should typically resolve within 15–20 minutes. Such changes are usually aided by increasing mean arterial blood pressure to promote improved spinal cord perfusion, as well as tem- porary cessation of further surgical maneuvers. More seri- ous concussive injury, such as those caused by an uncontrolled surgical instrument or pedicle screw impinge- ment upon the spinal cord, will obliterate both sensory and motor evoked potentials entirely. Spinal cord ischemia may result from: 1) stretching of critical spinal cord vascular supply during correctional ma- neuvers or placement of a cervical strut graft, 2) prolonged hypotension or 3) after ligation of anterior segmental ar- teries. Speed of corrective maneuvers also seems to play a role in developing myelopathy in this setting and is par- ticularly important relative to hemodynamic management. Maintenance of mean arterial blood pressure near normal promotes tissue accommodation to elongation while com- pensating for changes in spinal cord perfusion pressure. Prolonged hypotension, whether deliberate or systemic, can result in spinal cord vascular injury. TceMEPs are par- ticularly sensitive to blood pressure changes and can be used quite effectively to titrate how much of a hypotensive state the spinal cord will withstand. Because the motor and sensory components of the spinal cord are separated, and because the spinal cord blood supply is heterogeneic (ie, anterior and posterior spinal arteries), monitoring a single evoked potential modality may not reflect the global status of spinal cord function. The blood supply nourishing the posterior column sensory pathways which mediate SSEPs are the posterior spinal arteries. It is entirely possible to have selective loss of SSEPs with complete sparing of mo- tor function [35]. Conversely, selective ischemia of the an- terior spinal cord region, as in the case of anterior spinal artery syndrome, may manifest as a loss of MEP amplitude in the absence of concurrent change in SSEPs [36]. Based on the pathophysiology of spinal cord injury, combined monitoring techniques including somatosensory and transcranial electric motor evoked potentials, aug- mented by H-reflexes and F-responses, when possible, could facilitate rapid identification of either concussive or ischemic changes leading to spinal shock. The pathophysi- ology of spinal shock is thought to involve neuronal mech- anisms of inhibition and reorganization after acute spinal cord injury. Although not entirely clear, it may be mediated 216S V.J. Devlin et al. / The Spine Journal 6 (2006) 212S–224S
  6. 6. by synaptic changes in the segments of the cord below the site of injury. These changes seem to relate to heightened presynaptic inhibition and hyperpolarization of the spinal motor neurons. The H-reflex and F-response, respectively, seem to be exquisitely sensitive to these changes. As a re- sult, they may actually obliterate before a loss of tceMEPs, thereby serving as an early harbinger of neurological disas- ter. Yet, unreliable intraoperative recording of the H-re- flexes or F-responses, particularly in adults who often present with complex medical issues, precludes their rou- tine use and hence, lessens their overall value. Spinal nerve roots are also susceptible to injury due to mechanical or ischemic insult. Anatomically, the dorsal and ventral roots split into rootlets and mini-rootlets prox- imally. This division site is the location where the nerve root is susceptible to mechanical injury as the axons, en- closed by a thin root sheath, cerebrospinal fluid, and menin- ges, lack the protective covering of epineurium and perineurium that is present in peripheral nerves. There also seems to be an area of hypovascularity between the proxi- mal and middle third of the dorsal and ventral roots where the nerve is susceptible both to mechanical and ischemic in- sult [37]. If there is microtrauma leading to mechanical or metabolic nerve root irritation, the nerve root will depolar- ize, resulting in an action potential that leads to the release of acetylcholine and depolarization of the motor end plate from innervated muscle fibers. These mechanically pro- duced nerve action potentials can be identified, both visu- ally and acoustically, by monitoring electromyography as was discussed previously. Effects of anesthetics on neurophysiological signals The success of intraoperative monitoring is highly de- pendent on appropriate anesthetic management [38–41]. Paramount is the understanding that essentially all anes- thetic agents depress synaptic function both in brain and spinal cord gray matter. In spinal cord monitoring, the mar- gin for interpretation error is narrow because the signal am- plitudes are inherently quite small (ie, microvolt range). Any anesthetic that depresses signal amplitude will poten- tially lead to increased variability and interpretive ambigu- ity. When signal amplitude is artificially depressed and highly fluctuant as a result of anesthesia, it creates a situa- tion where signal change must be interpreted in the pres- ence of extreme clinical uncertainty. In general, all inhalational agents (isoflurane, desflurane, sevoflurane) produce a dose-related increase in latency and reduction in amplitude of the cortical SSEP. While the ex- act sites of action for these potent agents remain unclear, these gases appear to dissolve in the neuronal blood plasma membrane interfering with electrical function. The resulting effect is inhibition of ion channel function with significant alteration in synaptic and axonal transmission. As a result, neurophysiological signals that rely on synaptic function will be influenced to a far greater extent than signals that are not synaptically dependent. Even at steady-state, low end-tidal concentrations (0.25–0.5 mean alveolar concentra- tion (MAC)), response amplitude not only can become highly unstable and variable, but in many instances, either too small to detect a real change or completely obliterated. The effect of potent anesthetics is much less on the sub- cortical SSEP recorded over the cervical spine, compared with its cortical counterpart, and is minimal on spinal epi- dural or peripheral responses. In the past, when the only modality monitored was the SSEP, it was possible to over- come the adverse effects of inhalational anesthesia simply by recording a subcortical potential in the presence of mus- cle relaxation. The debilitating effect of volatile anesthetics on the excitability of cortical axons needed to elicit and re- cord transcranial motor evoked potentials is even more crit- ical today, given that the use of tceMEPs is increasing. The neural mechanism of tceMEP amplitude depression in the presence of a volatile anesthetic is due to the blockage of synaptic transmission both at the cortical and spinal ante- rior horn cells levels. There seems to be general consensus that nitrous oxide reduces cortical SSEP and tceMEP am- plitudes to a sufficiently great extent that it should be avoided entirely. This is particularly important if using a volatile agent because the introduction of nitrous oxide lowers the MAC, thereby having an additive effect on evoked potential amplitudes [42–46]. From the foregoing discussion it can be seen that when generation of a cortical signal, either ascending (SSEP) or descending (tceMEP) is necessary, as with contemporary spinal cord monitoring, it is best to avoid inhalational agents soon after induction and intubation in order to en- sure optimal amplitudes and unambiguous interpretation. On the basis of the unpredictable amplitude variability and depression associated both with volatile agents and with nitrous oxide, it has become routine in many high-vol- ume pediatric and adult spine surgery centers to use a total intravenous anesthetic regimen [47]. If total intravenous an- esthetic is precluded, one should begin surgery with a com- bined low level (eg, 0.3 MAC) volatile agent, augmented by combination intravenous drugs. Before the advent of multimodality techniques for mon- itoring of spinal cord and nerve root function, it was com- monplace to keep the patient fully relaxed. Neuromuscular relaxants have no adverse effect on SSEPs. In fact, a com- pletely relaxed patient reduces contaminating myogenic in- terference and fosters better SSEP waveform resolution, particularly for the subcortical response recorded over the cervical spine. Patient relaxation also eliminates any ‘‘shak- ing’’ of the upper and lower limbs secondary to peripheral nerve electrical stimulation. In contrast, neuromuscular blockade will compromise tceMEP, H-reflex, F-response, and EMG recordings, thereby introducing one more vari- able to cause interpretive ambiguity. While their has been some suggestion that myogenic tceMEP and EMG monitor- ing is possible in the presence of partial muscle relaxation, 217SV.J. Devlin et al. / The Spine Journal 6 (2006) 212S–224S
  7. 7. there are myriad factors that can lead to false-negative re- sults that were not considered in these limited sample stud- ies. For example, a definitive differential intramuscular and intra-side sensitivity to neuromuscular blockade has been noted. Thus, partial muscle relaxation may cause complete obliteration of one muscle group (eg, foot muscle), while sparing, at least to some degree, that of another (eg, intrinsic hand muscle). If, therefore, partial neuromuscular blockade was based on train-of-four testing from face or hand mus- cles, it may preclude tceMEP recordings over the lower ex- tremities and may result in a false-positive interpretation. Finally, one should not dismiss the interactive effects that volatile or intravenous anesthetics have on the pharmacody- namics and pharmacokinetics of nondepolarizing relaxants; that is, most of the anesthetic agents used will potentiate neuromuscular blockade. To this end, the use of partial neuromuscular blockade is simply too uncontrollable to justify use. Surprisingly little is known or understood about the in- fluence of anesthesia on spinal cord perfusion and metabo- lism as compared with that of the brain. During spine surgery, extrinsic pressure on the spinal cord such as stretching of the spinal cord vascular supply, mechanical compression of the spinal cord from tumor tissue, herniated disc, bone displacement, and the like, may potentially com- promise spinal cord perfusion pressure, thereby reducing spinal cord blood flow. When coupled with the somewhat unknown influences of anesthesia on spinal cord blood flow, it becomes critical to be vigilant to the patient’s blood pressure. There is experimental evidence to suggest that moderate hypotension can produce irreversible paralysis in a partially compressed, albeit functionally intact spinal cord, whereas mild hypotension can decrease spinal cord blood flow and electrical transmission. IONM data provide excellent clinical material to demonstrate the relationship between mean arterial pressure and spinal cord perfusion. It would appear that unlike the brain, spinal cord blood flow may not be as auto-regulated and thus, may be predisposed to ischemic injury when stressed during hypotension. State-of-the-art approach to neurophysiologic monitoring during surgery for cervical myelopathy Intraoperative neurophysiological monitoring plan Successful neurophysiologic monitoring requires the collaborative efforts of surgical, anesthesia, and moni- toring personnel. Patient-specific factors (eg, preoperative neurologic status, diabetes), procedure-related factors, and possible interventional measures that can be initiated to re- verse impending neurologic injury should be reviewed pre- operatively. Monitoring of ventral motor spinal cord tracts (tceMEPs), dorsal spinal cord sensory tracts (SSEPs), and motor nerve roots (EMG, tceMEPs) is performed from the time of anesthesia induction through emergence [48,49]. The surgical levels determine the neural structures at risk and guide the monitoring plan (Fig. 1). In upper cer- vical procedures (e.g., C1–C2), in addition to the risk of Cervical Monitoring Modalities Above C4? YES Risk of Vertebral Artery Injury? YES Monitor Brainstem Function BAER Upper SSEP Monitor Brachial Plexus TCeMEP Lower SSEP TCeMEP stEMG Pedicle Screws? YES NO spEMG Monitor Nerve Roots Monitor Lower Cervical Spinal Cord Monitor Upper Cervical Spinal Cord NO NO Fig. 1. Decision matrix to define what neural structures are at risk and which monitoring modalities should be used during cervical spine surgery. (From Schwartz and Sestokas [48].) 218S V.J. Devlin et al. / The Spine Journal 6 (2006) 212S–224S
  8. 8. spinal cord injury there exists risk of brainstem infarct sec- ondary to vertebral artery injury. SSEPs can be supple- mented with brainstem auditory evoked potentials to enhance monitoring of brainstem perfusion. In procedures above the level of C4, upper extremity SSEPs are generally adequate and lower extremity SSEPs are not required, be- cause both the median and ulnar nerves enter the spinal cord at spinal root levels below C4. Thus, these ascending potentials have no opportunity to bypass a site of injury via another spinal nerve root entry level. The combination of upper extremity SSEPs and tceMEPs recorded from upper and lower extremities provides excellent coverage of ven- tral and dorsal spinal cord tracts and can also be used to monitor for positional brachial plexopathy. In procedures below C4, the vertebral artery is not generally at high risk and emphasis shifts to monitoring for potential injury to cervical nerve roots, especially C5, using spEMG and tce- MEP recorded from the deltoid as well as hand muscles. If pedicle screws are used, stEMG is appropriate to assess pedicle wall integrity. Monitoring of cord function is per- formed with tceMEPs and SSEPs recorded from both upper and lower extremities. There exists a subset of patients with cervical myelopathy for whom SSEPs are poorly defined and unmonitorable. In many of these cases, tceMEPs can still be recorded. In a recent analysis [50] of spine surgery patients with unobtainable evoked potential data despite functional neural integrity, the incidence of absent data was extremely low in the population with degenerative spi- nal disease (0.38%) and highest in patients with neuromus- cular disease (6.8%). Intraoperative neurophysiologic monitoring sequence We initiate IONM immediately after induction in order to prevent exacerbation of spinal cord compression as a re- sult of neck extension [51,52]. Postinduction preintubation baseline tceMEPs are obtained. Then the anesthesiologist may begin intubation while monitoring is continued. After intubation, tceMEPs are obtained upon airway access be- fore taping the endotracheal tube. SSEPs are obtained as well during this time. SSEPs and tceMEPs are repeated after presurgical maneuvers including traction weight placement, shoulder taping, and position changes. If moni- toring changes are noted with weight placement or taping, the forces applied to the neck and upper extremities are de- creased. Improvement in monitoring potentials typically occurs within a few minutes of these corrective maneuvers. At our institution, if monitoring changes occur after turning the patient from supine to prone, the mean arterial blood pressure is raised to at least 90 mm in order to ensure ade- quate spinal cord perfusion. If tceMEPs and SSEPs do not improve (e.g., 30% improvement in tceMEP amplitude) within a short time (e.g., 15 minutes), then a spinal cord in- jury dose of methylprednisolone is considered and the pa- tient is turned supine while monitoring continues during emergence from anesthesia. After the patient is positioned and stable baseline neu- rophysiologic potentials are documented, cervical surgery can proceed. During anterior procedures, tceMEPs are documented during critical portions of the procedure including intervertebral distraction, implant or graft place- ment, neck extension, or application of additional traction weight. Posterior cervical procedures are associated with increased risk for neurologic injury to both the spinal cord and spinal nerve roots due to either mechanical or vascular etiology. Monitoring of tceMEPs is an option during lam- inectomy/foraminotomy, posterior implant placement, and other posterior surgical maneuvers. Monitoring of SSEPs is useful both as a back-up to tceMEPs for detection of compressive injury as well as for detection of compro- mised brainstem and spinal cord blood flow. Inadequate blood pressure may leave the spinal cord with insufficient reserve to withstand surgical manipulations and may be detected by careful intraoperative neurophysiologic moni- toring. Surveillance of the status of spinal nerve roots us- ing EMG and tceMEPs recorded from the deltoid and biceps is considered when the C5 and C6 nerve roots are at risk. Current state of the medical literature regarding neurophysiologic monitoring during surgery for cervical myelopathy and related disorders A computerized search of the database of the National Library of Medicine from 1996 through 2005 was con- ducted using the search terms ‘‘electrophysiology and spine/surgery’’ or ‘‘electromyography and spine/surgery’’ or ‘‘evoked potentials and spine surgery’’ yielding a total of 287 citations. Restricting the search to ‘‘electrophysiol- ogy and cervical spine/surgery’’ or ‘‘electromyography and cervical spine/surgery’’ or ‘‘evoked potentials and cervical spine surgery’’ yielded a total of 72 citations. Ref- erences relevant to the use of intraoperative neurophysio- logic monitoring during cervical procedures were broadly categorized according to whether they reported use of a sin- gle monitoring modality or multiple modalities for intrao- perative assessment of spinal cord function. References relevant to the use of monitoring for assessment of cervical nerve roots were also analyzed. Studies reporting the use of neurophysiologic monitoring techniques in relation to the detection of miscellaneous related intraoperative neuro- logic or vascular problems during spine surgery in general and cervical surgery in particular were reviewed separately. Spinal cord monitoring Somatosensory evoked potentials (SSEPs) Epstein et al. [53] compared the morbidity and mortality of 100 consecutive SSEP monitored cervical procedures with a historical control population of 218 patients who 219SV.J. Devlin et al. / The Spine Journal 6 (2006) 212S–224S
  9. 9. underwent unmonitored cervical procedures for myelopa- thy and radiculopathy. SSEPs were determined to be valu- able in improving patient outcome after cervical surgery in the monitored group. In the unmonitored group, 3.7% be- came quadriplegic and 0.5% died whereas no instances of quadriplegia or death were noted in the monitored group. The reduction in neurologic deficit was attributed to early detection of vascular or mechanical compromise of the spi- nal cord or nerve roots thereby permitting alteration of an- esthetic or surgical technique including reversal of hypotension, adjustment of operative position, release of distraction, and cessation of manipulation. However, the historical cohort design of the paper limits the conclusions which can be made regarding efficacy of intraoperative SSEP monitoring. May et al. [8] reported a series of 191 pa- tients undergoing cervical surgery for diverse diagnoses. Upper limb SSEP responses were recorded reliably in 182 patients with a sensitivity of 99% and specificity of 27% in 10 patients who developed neurologic signs postop- eratively. Potential risk factors for electrophysiologic and neurologic deterioration were determined as: 1) preopera- tive myelopathy; 2) long segmental extent of surgery; 3) upper cervical surgery; 4) use of instrumentation; and 5) application of corrective forces to the neck. Myelopathy alone was such a strong risk factor in this series that it over- shadowed the effect of the other recognized factors. It was argued that the false-positives in this series may have in- cluded a number of patients in whom neurologic deteriora- tion was successfully prevented as a consequence of the surgeon’s response to the report of SSEP amplitude loss. Kombos et al. [54] prospectively evaluated SSEP monitor- ing in 100 patients (Group 1dcervical myelopathy; Group 2dradiculopathy or mild hyperreflexia; Group 3dacute neurologic deficit) treated with anterior cervical decom- pression and fusion. SSEPs were performed during five stages of the procedure: M1, after induction of anesthesia; M2, during positioning; M3, during distraction of the inter- vertebral space; M4, throughout decompression; and M5, during graft placement. No SSEP changes were identified in any patients during induction (M1). Deterioration of SSEPs was seen across all groups (35% of all patients; 41% in Group 1, 23% in Group 2, 47% in Group 3) during positioning (M2) and 5 minutes afterward. Deterioration of SSEPs was reported with distraction of the intervertebral space (M3) in Group 1 (17%) and Group 3 (40%). No SSEP changes were noted with decompression of the spinal cord (M4) in any group. Acute deterioration of SSEPs was re- corded in one Group 2 patient during graft placement (M5). In this study, intraoperative SSEP monitoring during anterior cervical spine surgery permitted modification of surgical strategy to reduce the SSEP deterioration. The most common changes occurred during patient positioning and were more frequent in patients with myelopathy. Jones et al. [55] reported two cases of quadriparesis fol- lowing routine anterior cervical discectomy which were not detected with SSEP monitoring. They concluded that although the neurological risks associated with anterior cervical discectomy are generally considered to be low (0.1–0.4.6%), they are not negligible. Irrespective of the cause and level of neurologic deficit, they advocated com- bined monitoring of SSEPs and MEPs during anterior cer- vical discectomy. Sebastien et al. [56] reported SSEP data obtained during cervical procedures in 210 patients. SSEP changes were noted in 84 patients (40%) and were attrib- uted to mechanical stress (13 patients), regional ischemia (17 patients), and manipulation or placement of instrumen- tation (44 patients). No false-negatives were reported. Sloan et al. [57] reported three cases of SSEP loss during anterior cervical surgery attributed to retractor placement which was believed to have caused occlusion of the carotid artery. It was reported that intervention to reverse these monitoring changes prevented cerebral ischemia and conse- quent cortical damage. In contrast to the above studies, Taunt et al. [58] reported a series of 175 patients in which 163 patients were monitored with SSEPs from the median and posterior tibial nerves during anterior cervical surgery. Patient groups included radiculopathy (132 patients), mye- lopathy (30 patients), unstable cervical fractures (11 pa- tients), and cervical pain (2 patients). 96.3% of patients had no SSEP changes and no neurologic deterioration after surgery. Monitoring data were reported as showing three false-positives (1.8%) and one false-negative (0.6%). The single false-negative case was a right deltoid palsy not detected with median nerve stimulated SSEPs. It can be questioned whether this finding is accurately termed a ‘‘false-negative’’ as SSEPs cannot be expected to detect single nerve root deficits. Anterior cervical decompression and fusion was considered by the author to be a safe proce- dure with a low rate of complications for which intraoper- ative SSEP monitoring was not helpful. Transcranial electric motor-evoked potentials (tceMEPs) In 1989 Kitagawa et al. [59] reported the use of intrao- perative monitoring during upper cervical spine surgery in 20 patients. MEPs were produced by transcranial electrical stimulation and recorded from an epidural electrode. Five patients had transient attenuation of approximately 50% but experienced complete recovery after intraoperative adjustments by the surgeon, and none of these patients developed neurologic deficits postoperatively. One patient who developed complete loss of MEPs during surgery be- came a respiratory quadriplegic. There were no false-nega- tives. MEPs were sensitive to the operative procedure and responded within a few seconds to intraoperative adjust- ments. Gokaslan et al. [60] demonstrated that motor path- ways could be successfully monitored during anterior cervical surgery via MEPs obtained with a transcutaneous epidural electrode. The electrode placement was success- fully achieved in 15 patients but was unsuccessful in a sin- gle patient with cerebral palsy. No significant changes in 220S V.J. Devlin et al. / The Spine Journal 6 (2006) 212S–224S
  10. 10. MEPs occurred during surgery, and all patients had motor function at or above baseline after surgery. Although there remains lack of consensus about the optimal method for elicitation and recording of MEPs to electrical stimulation of the motor cortex, there is a growing consensus as to the value of this monitoring modality for both brain and spine surgery [61]. Nerve root monitoring Beatty et al. [62] described the use of continuous intra- operative EMG recording during spinal surgery. In a series of 150 cases, 30 cervical cases were monitored and EMG recording was reported to yield valuable information indi- cating when undue retraction was exerted on a nerve root or when a nerve root was adequately decompressed. How- ever, there was a 20–25% false-negative rate in which no firing was obtained with nerve root retraction. Jellish et al. [63] described intraoperative EMG monitoring of the posterior pharynx as a surrogate for monitoring recur- rent laryngeal nerve function. Recurrent laryngeal nerve function has also been monitored by use of a special elec- trified endotracheal tube (Medtronic Xomed, Jacksonville, FL) which serves as the recording device [64]. Postoperative C5 nerve root palsy remains problematic in patients who undergo cervical decompressive proce- dures. The average reported incidence of postoperative C5 palsy is 5.6% in cervical spondylotic myelopathy pa- tients, 8.3% in patients with ossification of the posterior longitudinal ligament, and literature review demonstrates that the incidence of this complication does not vary signif- icantly according to whether an anterior or posterior surgi- cal approach is used [65]. Postoperative C5 nerve root palsy remains incompletely understood and is considered to have multiple potential etiologies. In some reports, traumatic surgical technique or a tethering effect induced by exces- sive migration of the spinal cord after decompression are considered causative factors. Upper trunk brachial plexus injury resulting from positioning or intraoperative traction is an additional etiology. Impairment of autoregulation in the spinal cord gray matter has been suggested to play a role in cases where the onset of paralysis is delayed and occurs in the postoperative period [66]. Sasai et al. [67] reported that preoperative electromyography was a sensitive predic- tor of postoperative C5 palsy after laminoplasty. Treatment of preexistent subclinical C5 root compression with selec- tive foraminotomy in addition to posterior central canal de- compression could potentially avoid this complication in select patients. Routine intraoperative neurophysiologic monitoring (IONM) of upper extremity SSEPs, DEPs, and tceMEPs recorded from hand muscles generally has failed to detect this serious complication. In an effort to reduce the incidence of postoperative C5 nerve root injury, intrao- perative deltoid and biceps tceMEP and spontaneous EMG monitoring has been reported [26]. These two neurophysi- ologic methods are considered to play complementary roles for early detection (spEMG) and functional assessment (tceMEPs) of C5 nerve root injury during posterior cervical decompressive procedures. Recently, Jimenez et al. [68] reported the value of continuous C5 EMG monitoring in identifying and preventing postoperative C5 palsies during cervical surgery. A prospective cohort of 161 patients monitored with both spontaneous and triggered EMG tech- niques, SSEPs, and tceMEPs were compared with a histori- cal control group of 55 patients monitored without use of EMG techniques. With comparison to the control group, the incidence of postoperative C5 palsy was decreased from 7.3% to 0.9% with monitoring. Unfortunately, the historical cohort study design precludes any firm conclusions regard- ing the role of monitoring in this observed decrease. Nerve root monitoring may potentially play a role in recognition of intraoperative C5 root impairment caused by direct trauma or positioning but is not currently helpful in predict- ing delayed onset root impairment which develops during the postoperative period. Multimodality IONM Hilibrand et al. [13] reported a series of 427 consecutive cervical spine cases (324 anterior procedures, 83 posterior procedures, 20 combined anterior and posterior procedures) in which multimodality IONM was performed. Fifty-one percent (216 patients) underwent surgery for cervical spon- dylotic myelopathy and 10% (22 patients) had ossification of the posterior longitudinal ligament. SSEPs and tceMEPs recorded from hand muscles were performed in all patients during surgery. Twelve patients developed substantial or complete loss of amplitude of the tceMEPs. Ten of these patients had complete reversal of amplitude loss with prompt intervention (increasing mean arterial pressure, re- moval of anterior bone graft) while two patients awoke with a new motor deficit after surgery. SSEPs failed to identify any changes in one of these two patients; SSEP changes lagged behind tceMEP changes by 33 minutes in the other patient who awoke with neurologic deficit. In patients who had major potential changes detected by both SSEPs and tceMEPs, the SSEP changes lagged behind the tceMEP changes by an average of 16 minutes. This delay would re- duce the window of opportunity for intervention and could theoretically prevent or compromise reversal of spinal cord injury if patients were only monitored with SSEPs. In this study, tceMEPs were 100% sensitive and 100% specific, whereas SSEPs were only 25% sensitive but 100% specific. In addition, all except one of the 12 patients who developed tceMEP changes underwent surgery for cervical spondy- lotic myelopathy and four of these patients also presented with ossification of the posterior longitudinal ligament. Based on this study, the authors strongly recommend use of both tceMEPs and SSEPs when operating on patients with cervical spondylotic myelopathy in general, especially if they have ossification of the posterior longitudinal ligament. 221SV.J. Devlin et al. / The Spine Journal 6 (2006) 212S–224S
  11. 11. Bose et al. [69] reported a series of 119 patients treated with instrumented anterior cervical fusion primarily for radiculopathy who underwent IONM with SSEPs and tce- MEPs. Six neurophysiologic alerts prompted surgeon or an- esthesiologist intervention. Two alerts were related to sudden hypotension, three alerts were the result of neck hy- perextension, and one was the result of arm positioning during surgery. Three patients awoke with new motor weakness after surgery. One was correctly predicted by monitoring, but deficits were not detected in two patients. One patient developed additional postoperative weakness of the C5–C6 nerve roots which could not be detected dur- ing surgery because of absent baseline tceMEPs from the affected muscles. One patient developed quadriparesis which could not be detected with monitoring owing to ex- cessive use of neuromuscular blockade during surgery. The authors suggested that intraoperative monitoring provided valuable information during patient positioning especially with respect to the degree of neck extension. In addition, they reported that conventional criteria for assessing ade- quacy of blood pressure do not address the question of spi- nal cord perfusion directly as they do not take into account preexisting vascular compromise of the spinal cord. Fan et al. [26] described refinement of the standard multimodal- ity monitoring protocol (tceMEPs, SSEPs, spEMG) to in- clude tceMEPs recorded from deltoid and biceps in a series of patients undergoing cervical laminectomy for myelopathy in an effort to identify impending C5 nerve root injury. Conclusions In patients with cervical myelopathy, the spinal cord is already compromised to a point at which there is little re- serve for surgical maneuvers and the slightest adverse ac- tion can result in dramatic consequences. The changes induced by cervical spondylotic myelopathy are mainly is- chemic in origin. The same ischemic mechanism likely ac- counts for the deterioration noted in patients in whom corrective forces are applied in the course of surgical treat- ment. These forces, rather than direct cord injury, are prob- ably responsible for the observed electrical changes when the head and neck are manipulated during positioning or during intraoperative maneuvers that involve graft/cage placement, correction of kyphosis, or alteration of spinal alignment via spinal instrumentation. These maneuvers are associated with vascular stretching which may ulti- mately lead to compromise of spinal cord function. Hypo- tension is an additional factor which may lead to irreversible neurologic deficit in a partially compressed but functionally intact spinal cord. IONM data potentially provide useful data regarding such changes in the surgical patient. Medical evidence exists to support the validity of neuro- physiologic monitoring as a diagnostic tool for assessment of neurologic function during cervical spine surgery. When such information is desired, recording of both SSEPs and tceMEPs should be performed as they provide complemen- tary information and monitor different spinal cord tracts. According to the tenets of evidence-based medicine, in the absence of a randomized controlled trial regarding use of IONM during cervical surgery, statements regarding its efficacy in improving neurologic outcomes after surgery for cervical myelopathy are not conclusive, and recommen- dations regarding use of IONM only represent options and do not reflect any consensus guideline or standard of care. 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